1. Field of the invention
This invention relates in general to running tools for running a casing hanger in a subsea well, and in particular to a running tool that is run on casing and has a full bore inner diameter that equals the inner diameters of the running casing and well casing.
2. Description of the Prior Art
In a subsea well, a wellhead housing will be located at the sea floor. As the well is drilled to successive depths, casing will be run and cemented in place. A typical technique for running casing involves securing the casing to a casing hanger, then securing the casing hanger to a running tool. The running tool secures to drill pipe. The operator lowers the string of well casing, casing hanger, and running tool on the drill pipe.
A cement plug will be positioned below the casing hanger and supported by the running tool. Once the casing hanger lands in the wellhead housing, the operator will pump cement down the bore of the drill pipe. The cement passes through the plug and flows down the well casing, then back up the annulus surrounding the casing.
After the desired amount of cement has been pumped into the drill pipe, the operator will pump a ball or dart down the bore of the drill pipe. The ball will contact the plug and push the cement plug down to the bottom of the casing. The cement plug then locates at the upper level of cement, and forces all of the cement out of the well casing and up into the annulus.
The running tool frequently will have a seal sleeve mounted to it which carries a casing hanger seal. After cementing, the operator will actuate the seal sleeve, moving it downward relative to the casing hanger to position the seal between the casing hanger and the wellhead housing bore. In some cases, the running tool will also set the seal to the desired force. A variety of different structures have been known in the prior art to move the seal sleeve down. These structures include hydraulic systems that are actuated by pulling up on the drill pipe or moving the drill pipe downward. The systems also include torque setting by rotation of the drill pipe, or pumping fluid down the drill pipe.
Running tools of this nature work sufficiently well. However, one drawback is that with a very long string, the weight of the string of casing may exceed the strength of the string of drill pipe. Also, the drill pipe has an inner diameter that is much smaller than the inner diameter of the well casing. The cement plug must initially be located below the drill pipe during the running in procedure, which has some disadvantages.
Full bore casing hanger running tools are available for elastomeric casing hanger seals of some types. In the prior art type, the casing hanger seal is initially carried on the exterior of the casing hanger by threads. Flowby slots on the casing hanger allow the bypass of cement returns when cementing. After cementing, the running tool will engage the seal assembly and through rotation will move the seal downward into position. The seal is set by torque. While this works well enough, securing the seal assembly by threads to the casing hanger requires a longer casing hanger than running tools which carry the seal on a seal sleeve mounted to the running tool.